Restrict file access -- only read through PHP

Question

I am using a GoDaddy web hosting plan on a Windows platform. This was not my choice -- it has to do with a different part of the actual site using ASP.NET (also not my choice).

I have a SQL database with a bunch of entries with some non-sensitive customer information. The primary key on this is an AutoIncrement integer, and I have a series of PDF files that match up with each of those integers (e.g. 555.pdf, 7891.pdf, etc).

My goal is to restrict direct access to these files, I want users to have to go through a search and login process (PHP) first. Originally I planned to put the files above the PUBLIC_HTML folder, but GoDaddy refuses to give me root access without a dedicated server ($20 a month from them).

The next thing I looked into was HTACCESS. I was going to restrict access to the files to only PHP scripts by only allowing access to the Server's IP Address (or localhost/127.0.0.1). Unfortunately this doesn't work because GoDaddy does not run Apache on its Windows servers.

I could put the files into BLOBs in the database, but that gets really messy when I need to work with them quickly (plus I have had some trouble with that approach).

Any suggestions to restrict access to the files only to a PHP script (readfile())?

Answer 1

Since you can't put the files anywhere but in your public_html directory, you'll have to go for the feared/hated "security by obscurity" method

1. Create a randomly named sub-directory to store the files in: public_html/RANDOMGARBAGE

2. Make sure the directory is not browseable. Disable directory browsing (if you can), and put a default document (index.html?) in there as well, so even if browsing is on, you won't get the directory listing.

3. Don't store your files with guessable names. Instead of storing them with the database ID, store them with a salted+hashed name instead: $crypted_filename = sha1($real_filename . 'some hard-to-guess salt text'); (of course, make this more complex if you need to). Store the original filename in your database. So you end up with something like:

   public_html/RANDOMGARBAGE/5bf1fd927dfb8679496a2e6cf00cbe50c1c87145 public_html/RANDOMGARBAGE/7ec1f0eb9119d48eb6a3176ca47380c6496304c8

4. Serve up the files via a PHP script - never link to the hashed filename directly

   Download

which then does:

<?php

    $fileID = (int)$_GET['fileID'];

    $crypted_file = sha1($fileID . 'some hard-to-guess salt text');

    $full_path = 'public_html/RANDOMGARBAGE/' . $crypted_file;
    if (is_readable($full_path)) {
         if(user_is_allowed_to_see_this_file()) {
             /// send file to user with readfile()
             header("Content-disposition: attachment; filename=$ORIGINAL_FILENAME");
             readfile($full_path);
         } else {
             die("Permission denied");
         }
    } else {
        /// handle problems here
        die("Uh-oh. Can't find/read file");
    }

This way the user will never see what your "s00per seekrit" filename is, they'll just see their browser hit ...php?fileID=37 and start a download of secret file.pdf

On top of this, you can occasionally rename the special sub-directory to something else on a regular basis, as well as change the salt text (which then requires you update all the hashed filenames with the new sha1 values).